1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2010-096782 discloses a fixing device including a heating unit. The heating unit includes a pipe-like metallic heat conductor disposed so as to face an inner peripheral surface of a fixing belt serving as a fixing member, and a heater which heats the metallic heat conductor. The heating unit heats the entirety of the fixing belt through the heating of the metallic heat conductor. Further, the fixing device disclosed in Japanese Patent Application Laid-open No. 2010-096782 includes a nip forming unit which is disposed near the inner peripheral surface of the fixing belt and comes into press-contact with a pressing roller serving as a rotationally driven pressing member with the fixing belt interposed therebetween so as to form a fixing nip. The driving force is transmitted to the fixing belt by the friction against the pressing roller, so that the fixing belt slides on the outer peripheral surface of the metallic heat conductor.
Since both ends of the nip forming unit in the width direction of a recording sheet (hereinafter, referred to as a main-scanning direction) are fixed and supported by the side plates of the fixing device, the center of the nip forming unit in the main-scanning direction may be largely bent in a direction in which the center moves away from the pressing roller due to the pressure which is applied from the pressing roller. As a result, both ends in the main-scanning direction of a contact surface of the nip forming unit, which comes into contact with the pressing roller with the fixing belt interposed therebetween, protrude outward and the center is pulled inward, thereby causing a phenomenon in which a nip width formed by the pressing roller and the nip forming unit is largely narrowed at the center compared to at the end in the main-scanning direction. As a result, there is a problem in that unevenness of an image occurs at the center and at the end in the main-scanning direction or a conveying failure occurs.
For this reason, in the fixing device disclosed in Japanese Patent Application Laid-open No. 2010-096782, the nip forming unit is formed so that the center of the contact surface of the nip forming unit in the main-scanning direction protrudes toward the pressing roller in relation to both ends in the main-scanning direction in no-load state. As a result, when the nip forming unit is bent, the contact surface of the nip forming unit becomes flat. That is, in this related art, the center of the contact surface of the nip forming unit in the main-scanning direction protrudes toward the pressing roller by a bent amount. Accordingly, when the nip forming unit is bent by the pressure of the pressing roller, the contact surface of the nip forming unit may be made to be flat, and a deviation between the end and the center in the main-scanning direction may be solved.
However, in the fixing device disclosed in Japanese Patent Application Laid-open No. 2010-096782, a problem is found in which a defective image is obtained or a conveying failure occurs depending on the type of a sheet which is conveyed to the fixing nip, for example, a recording sheet having a smooth surface. The inventors could find out the following result by carefully studying the problem. That is, in a case where the recording sheet is conveyed to the fixing nip, a sheet passing portion which comes into contact with the recording sheet at the center of the fixing belt in the main-scanning direction comes into contact with the surface of the recording sheet, and the rotational driving force of the pressing roller is transmitted to the sheet passing portion with the recording sheet interposed therebetween. However, in the recording sheet having a smooth surface, the friction between the recording sheet and the pressing roller or the friction between the recording sheet and the fixing belt decreases, so that the rotational driving force which is transmitted from the pressing roller to the fixing belt decreases. Further, in non-sheet passing portions at both ends which directly come into contact with the pressing roller in the main-scanning direction, the contact pressure decreases due to the thickness of the recording sheet. Accordingly, a sufficient friction may not be obtained even from the non-sheet passing portions, so that the rotational driving force which is transmitted from the pressing roller to the fixing belt decreases. As a result, it has been found that the driving force which is transmitted from the pressing roller to the fixing belt decreases, and the sliding resistance against the outer peripheral surface of the metallic heat conductor increases, so that the fixing belt slips producing the unevenness of the image or the fixing belt stops its rotation causing a conveying failure of the recording sheet.